Arcograph



July 2,1935. F JQHNSQN 2,006,769

ARCOGRAPH Filed June 1, 1933 Patented July 2 1935 UNITED STATESARCOGRAPH Frank N. Johnson, incinnati, Ohio Application June 1, 1933,Serial No. 673,869

4 Claims.

My invention relates to an arcograph or an instrument for describingarcs of circles without reference to a center point. The instrument ofthe instant application belongs to that class of arcographs whichemploys two intersecting straight lines which are fixed by the fiducialedges of two straight edges, the lines being kept constantly in contactwith two fixed points on a circular curve in order to make the point ofintersection describe the curve when the latter point is moved from oneof the fixed points towards the other. These devices are mostadvantageously used in scribing circles having extremely long radii.

One of the objects of my invention is to pro-- vide an arcograph ofsimplified. constru tion which can be used withextreme facility. This isaccomplished partly by employing two straight edges rotatable withrespect to each other about a fixed point and so constructed andassembled as to eliminate all interferences to the vision or to theoperation of a scribe tool. It is a further object of the invention tomaintain the axis of rotation in the lines of the fiducial edges of thestraight edges and to point out the location of this axis. A furtherobject is to provide novel means for readily maintaining a manuallymanipulated scribe tool in proper position.

In order to accomplish these and other objects of the inventionarcograph is comprised of a pair of straight edges rotatable withrespect to each other about an axis with means for maintaining the edgesin predetermined relative positions and in forming one fiducial edgecontinuous through the intersection of the lines of the fiducial edges.I

In the particular embodiment of my invention selected for illustration:

Fig. l is a top plan view of my device,

Fig. 2 is a bottom plan view of same,

Fig. 3 is an elevation of the device, part of the straight edges beingbroken away,

Fig. 4 is a diagrammatic view illustrating the operation of my device, 7

5 is an enlarged section on the line t ll of Fig. l, and

Fig. 6 is an end view partly in section and partly broken away.

Referring now to the drawing:

The device is comprised of coacting straight 50 edges l9 and II havingworking or fiducial edges l2 and I3 respectivelyf Near its inner end thestraight edge H is preferably provided with a plate l4 secured to thestraight edge in any desirable manner such as by means of brazing. Theplate I 4 is provided with upwardly extending lugs and It and carries ascrew threaded aperture l I for the purpose hereinafter described.

Near its inner end the straight edge i0 is provided with a plate it andintermediate plate I 9 se cured to the straight edge by screws 28 or inany 5 other suitable manner. The plate i 6 is preferably provided withan arcuate slot 29 in which the lugs I5 and N5 of the plate M are guidedso as to permit rotation of the straight edges ii] and H about a fixedaxis with respect to each other. A clamp screw has its threaded end passthrough the slot 2i and is received in the threaded aperture ll so thatits shoulder 24 seats against the plate is maintaining the straightedges in a predetermined relative position.

The straight edges l0 and H are both at their inner ends preferablybeveled rearwardly and outwardly, the straight edge ll being providedwith a nose 25 which slides under the lip 25 on the straight edge 5 6.Raising of the straight edge I I above the level of the under face ofthe straight edge ill permits movement of the instrument over the paperin scribing a curve so that no part of the curved line made is contactedby the instrument, so that the ink slot of a ruling pen if used as ascribing tool is free of contact with, the fiducial edge so as not tosmudge the paper.

The instrument is preferably provided with a scribe tool guide 2? whoseend preferably extends beyond the fiducial edge It. The member 2! issecured for rotation on the top of the plate l8 by means of a clampscrew 23 which passes through the guide 2? and islscrew threaded intothe plate 58. The upper face of the straight edge ii is preferablyprovided with an index mark 29 at the axis of rotation of the two edgesand the guide 2? is set in the desired position to maintain the scribetool point at the index mark in scribing an arc. (The scribe tool guide27 is preferably made adjustable since with different shapes and sizesof pens and pencils used in scribing the arcs, the points will notalways touch the paper when held against the guide, if it is fixed oralways remains in the same position. It is obvious that if a thin stylusand a broad ruling pen were both held at the same angle against thescribe tool guide, their points would touch the drawing paper atdifferent places and consequently the guide must be moved to a positionnecessary to cause the pen point to touch the paper near the index mark29.)

To facilitate the drawing of arcs I have provided novel tacks 3t each ofwhich is provided with a flange 3! and points 32 and 33 extending inopposite directions at right angles to the flange so as to permit theend of the plate l8 to pass over them in scribing an arc. Of course, oneend of the tack holds the work sheet to the drawing board and the otherend acts as a guide for a fiducial edge of the instrument.

When employing the preferred form of my device, and assuming that acurve X is to be drawn, suitable points in the curve are platted in anypreferred manner. as A, H and Gare platted, as at the ends of chords AH,GH and AG, the curve may be readily produced by inserting one of thetacks30 in the drawing board at each of the points A and G, and

adjusting the straight edges into such angular relation that the workingedges of the respective 29 at the fiducial edge of the straight edge Iis coincident with the point H. The parts are clamped in this relation,by tightening the clamp screw 22. This places the fiducial edges of thestraightedges in coincidence with the chords .AH andGH of the curve tobe produced.

V The guide Z'iis thereupon adjusted, so that the point .of the scribinginstrument immediately above its exetreme lower end rests against theworking edge of the straight edge atithe index 2,9,.which is raisedslightly above the paper or material, with a part of the scribinginstrument adjacentto and above said point resting against the contactedge of theguide 21, the scribing tool being manually slanted inselective position for most convenient manipulation by the operator,

and for proper relation of the scribing point with the paper or materialso as to produce a continuous curve by movement of the working edges ofthe straight edges along the pins at the points AG, which will cause thescribing of a curve along the curve X throughout the range of contact ofthe respective straight edges with the pins at the points AG, and permitthe passage of the plate I 3 l8 over the pin at the point G, whereby thefull measure of the, curved line may be produced.

After the curved line from A to G is scribed, other pointsalong thecurve may be found and the pins accordingly set, and the angularrelations between the straight edges adjusted for coincidence of theindex with the intermediate point found, as hereinbefore described. Ifthe new points found are equidistant with the previous points used alongthe curve, further adjust- The device itself is useful for locatingsuchpoints.

Points outside or inside the limits of the are which has been assumed tobe of convenient length may be located by the use of auxiliary straightedges held in contact with the fiducial edges. If the device be adjustedto describe the If three points in the curve,

are AG then the index will always indicate a point on this curve,whether it be outside or inside the arc AG, provided that the line ofone fiducial edge passes through A and the other through G. If thedistance AG be inconveniently long, supplementary intermediate pointsmay thus be located. Moreover, if the device be: adjusted to describethe arc AG, then iflthe index be at tioned fiducial edge be brought intothe line of this tangent, then the other fiducial edge will cross thearc produced, at a point distant from the index by the length AG.

Concentric curves may readily bedrawn with this instrument withoutresetting the device by offsetting the pins by equal intervals alongtheir respective radial lines.

While the invention has been described above in its preferred form, itwill be obvious that various changes and modifications can be madetherein without departing from the spirit or scope thereof and it istherefore desired that only such limitations be imposed thereon as arenecessitated by the prior art or specifically set forth in the claims. VW

I claim:

1. An arcograph having in combination a pair of straight edges, lugs onone of said straight edges near one of its ends, a slot on the other ofsaid straight edges coacting with said lugs to guide the relativeangular movement of said straight edges and means for maintaining saidstraight edges in a predetermined angular relationship.

2. An arcographhaving in combination a pair of straight edges, means forrotating. said straight edges with respect to each other about a fixedaxis, an index on one of said straight edges on the axis and anadjustable scribe tool guide extending beyond said straight edgesadjacent the index.

3. In an arcograph, the combination of a pair of straight edges providedwith working edges, a plate provided with an arcuate guide at the innerend of one of said straightedges, a coacting guide at the inner end ofthe other of said straight edges, clamping means for clamping saidstraight edges in adjusted angular positions, one of said straight edgesprovided with an index coincident with the centerof the arcs of saidarcuate guides, said center being in the line of said working edges, anda scribe tool guide adjacent tov said index, and arranged for manuallyguiding a scribe tool maintained at said index. I

4. An arcograph having in combination a pair of straight edges, a platesecured to one of said straight edges and having an arcuate slottherein, lugs on the other of said straight edges coacting (ill with theslot for rotating said straight edges in relation to each other about afixed axis and clamping means for maintaining said straight edges in apredetermined angular relationship.

' FRANK N. JOHNSON.

